Lithium secondary battery

ABSTRACT

In a lithium secondary battery comprising a chargeable and dischargeable positive electrode, a chargeable and dischargeable negative electrode and an electrolyte, wherein at least one of the positive electrode and the negative electrode comprises a lithium-containing halide having a spinel structure or spinel analogous structure, the lithium-containing halide having a spinel structure or spinel analogous structure having a high ion bonding property is dissolved into an electrolyte obtained by dissolving a salt into an organic solvent. In the secondary battery of the present invention, since a lithium ion conductive inorganic solid electrolyte is used as an electrolyte, there can be obtained a chargeable and dischargeable lithium battery in which at least one of a positive electrode and a negative electrode comprises a lithium-containing halide having a spinel structure or a spinel analogous structure.

TECHNICAL FIELD

The present invention relates to a lithium secondary battery using asolid electrolyte, and a halide having a spinel structure or a spinelanalogous structure as an active material.

BACKGROUND ART

Recently, with the development of portable appliances such as personalcomputers and portable telephones, demands for batteries as an electricpower source are extremely large. Particularly, since lithium has asmall atomic weight and is a material having large ionization energy, alithium battery is intensively researched in various fields as a batterywhich can obtain high energy density.

In electrodes of a battery, charge-transfer occurs between an ion movingin an electrolyte and a current flowing in an outer circuit. Therefore,as an active material in a lithium battery, a mixed conductor ispreferably used which has lithium ion conductivity together withelectron conductivity. One example of such a lithium ion-electron mixedconductor includes various compounds having a spinel structure. Acompound of a spinel structure has a three-dimensional diffusion pathfor a lithium ion and has a preferable structure for transfer of alithium ion.

Further, when a battery is charged and discharged, the amount of alithium ion in an active material changes, and with this change, thevolume of the active material also changes. When this voluminal changeis too large, deteriorations in battery properties are caused such as adeterioration in contact condition between active materials duringoperation of a battery. LiCoO₂ used presently as a positive electrodeactive material for a lithium secondary battery has a two-dimensionalstructure in which a lithium ion is present between layers formed byCoO₂. When the amount of lithium ion s between these layer s changes,the interlayer distance changes significantly. Whereas, a compoundhaving a spinel structure has a three-dimensional structure, and when itis utilized for an active material of a battery, such a voluminal changeconcurrent with charging and discharging can be suppressed.

As the compound having a spinel structure that has been investigated asan active material for a lithium battery. there are exemplified oxidessuch as LiMn₂O₄ and Li_(4/3)Ti_(5/3)O₄, and sulfides such asCu_(p)Ti₂S₄.

Additionally, as the lithium-containing compound having the same spinelstructure, there have been reported halides such as chlorides andbromides. These are, for example, Li₂MnCl₄ (C. J. J. van Loon and J. deJong, Acta Crystallog raphica B, 24, 1968 (1982)), Li₂FeCl₄ (R. Kanno,Y. Takeda, A. Takahashi, O. Yamamoto, R. Suyama, and S. Kume, Journal ofSolid State Chemistry, 72, 363 (1988)), Li₂CrCl₄ (R. Kanno, Y. Takeda,A. Matsumoto, O. Yamamoto, R. Suyama, and S. Kume, Journal of SolidState Chemistry, 75, 41 (1988)), Li₂CoCl₄ (R. Kanno, Y. Takeda, and O.Yamamoto, Solid State Ionics, 28, 1276 (1988)), Li_(2−2p)Mn_(1+p)Br₄ (R.Kanno, Y. Takeda, O. Yamamoto, C.Cros, W. Gang, and P. Hagenmuller,Journal of Electrochemical Society, 133, 1052 (1986)), and the like.

However, since these lithium-containing halides having a spinelstructure has a high ion bonding property, when an electrolyte preparedby dissolving a supporting electrolyte in a usual organic solvent isused, the halides are easily dissolved in the electrolyte. That is, itwas difficult to use the above-mentioned halides as an active materialin a lithium battery.

The present invention solves the above-mentioned conventional problems.Specifically, the object of the present invention is to provide alithium secondary battery using a lithium-containing halide having aspinel structure or a spinel analogous structure.

DISCLOSURE OF THE INVENTION

The present invention relates to a lithium secondary battery comprisinga chargeable and dischargeable positive electrode, a chargeable anddischargeable negative electrode and a lithium ion conductive inorganicsolid electrolyte, wherein at least one of the positive electrode andnegative electrode comprises a lithium-containing halide having a spinelstructure or a spinel analogous structure.

The above-mentioned lithium-containing halide is preferably representedby the formula Li_(2−2p−q)Me_(1+p)X₄, wherein Me is at least onetransition metal element selected from the group consisting of Ti, V,Cr, Mn, Fe, Co, Ni and Cu, X is at least one halogen element selectedfrom the group consisting of F, Cl, Br and I, and 0≦p≦0.5 and 0≦q≦2−2p.

The above-described lithium-containing halide is further preferablyrepresented by Li₂MeX₄, wherein Me is Fe, Mn or Co.

The above-described lithium ion conductive inorganic solid electrolyteis preferably a sulfide-based lithium ion conductive inorganic solidelectrolyte.

The above-described lithium ion conductive inorganic solid electrolyteis preferably a silicon-containing sulfide-based lithium ion conductiveinorganic solid electrolyte and preferably contains oxygen in an amountof 5 to 70 mol per 100 mol of silicon.

By use of a lithium ion conductive inorganic solid electrolyte as anelectrolyte, there can be obtained a rechargeable lithium battery usingas an active material a lithium-containing halide having a spinelstructure or a spinel analogous structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a lithium secondary battery in accordancewith one example of the present invention.

FIG. 2 is a charging and discharging curve graph shown by a lithiumsecondary battery in accordance with one example of the presentinvention.

FIG. 3 is a sectional view of a lithium secondary battery in accordancewith one comparative example of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention is a lithium secondary battery constituted by alithium ion conductive inorganic solid electrolyte, and an electrodecomprising as an active material a lithium-containing halide having aspinel structure or a spinel analogous structure.

In an inorganic solid electrolyte, only one kind of ion is transmitted.Namely, in a lithium ion conductive inorganic solid electrolyte, only alithium ion moves. Therefore, also when a lithium-containing halide witha high ion bonding property is used as an active material, the structureof the halide is kept and a rechargeable lithium battery can bemanufactured.

Meanwhile, an organic solvent electrolyte used in a usual lithiumbattery, or a solid polymer electrolyte even if it is a solidelectrolyte, does not have such ion selectivity. For example, when asolid polymer electrolyte is used, dissolution of a lithium-containinghalide scarcely occurs only if the electrolyte is in contact with thehalide as the active material. However, during operation of a battery,also an ion of the halide moves in the electrolyte, and as a result, thestructure of the lithium-containing halide can not be kept. Namely, itis difficult to manufacture a rechargeable lithium secondary battery.

As the lithium-containing halide in the present invention, a halidehaving a spinel structure or a spinel analogous structure which allowsexcellent diffusion of lithium ions is preferable, and particularlypreferable is a halide of tetragonal structure in which the moving rateof a lithium ion is high.

The lithium-containing halide having a spinel structure in accordancewith the present invention is represented for example by AB₂X₄.

Herein, the spinel structure includes an inverse spinel structure. Thespinel analogous structure is a structure wherein, in a skeleton havinga spinel structure represented by AB₂X₄, an A element or B element ispartially deficient or an A element site is partially substituted with aB element site.

In this case, X is a halogen ion, A represents the center position ofthe tetrahedron carrying halogen ions at the apexes, and B representsthe center position of the octahedron carrying halogen ions at theapexes. A is occupied mainly by a lithium ion, and B is occupied by alithium ion, a transition metal element and the like. As the transitionmetal element occupying B, transition metal elements which tend to causechange in valency concurrent with charging and discharging of a batteryare preferable, and specifically, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and thelike are preferable.

As the spinel type lithium-containing halide, there are exemplifiedchlorides such as Li₂FeCl₄ (A is occupied by Li, and B is occupied by Liand Fe in a molar ratio of 1:1), Li₂CrCl₄, Li₂VCl₄, Li₂TiCl₄, Li₂CoCl₄and Li₂MnCl₄, bromides such as Li₂CrBr₄ and Li_(2−2p)Mn_(1+p)Br₄,iodides such as Li₂CoI₄, fluorides such as Li₂NiF₄, those containing aplurality of halogen ions such as Li₂MnCl₄-Li₂MnBr₄ solid solutionsystem, as well as other compounds.

As the lithium-containing halide of the spinel analogous structure,there are exemplified Li_(1.6)Cu_(0.4)MnCl₄ in which an ion other than alithium ion is present at the center position of tetrahedron,Li₂Fe_(1−p)Cd_(p)Cl₄ in which other kind of element is present at thecenter position of octahedron, and the like.

The lithium-containing halide in the present invention is synthesized,for example, by mixing MeX₂ with LiX and heating the mixture.

The mixing ratio of MeX₂ and LiX may be selected according to thecomposition of the lithium-containing halide to be obtained. Forexample, if MeX₂ and LiX are mixed at 1:2 (molar ratio), Li₂MeX₄ havingan inverse; spinel structure can be synthesized.

The formal valency of an element (Me) in this halide is 2. When Me has avalency of 2, the lithium-containing halide in the present invention canbe generally represented by the formula: Li_(2−2p)Me_(1+p)X₄.

Namely, in the method of synthesizing Li₂MeX₄ having an inverse spinelstructure, if the mixing molar ratio of MeX₂ and LiX is from 1:2 to(1+p):(2−2p), Li_(2−2p)Me_(1+p)X₄ having a spinel analogous structure isobtained.

Herein, p is in the range of 0≦p≦0.5 wherein the ratio of the lithiumion and Me, both occupying B, is from 1:1 to 0:1.

Further, the lithium-containing halide is also represented by thecomposition: Li_(2−2p−q)Me_(1+p)X₄, since it can be oxidized in alithium battery.

Furthermore, if MeX₃ containing a transition metal element having aformal. valency of 3 is used instead of MeX₂ as a starting material inthe above-mentioned method, a lithium-containing halide represented byLi_(2−2p−q)Me_(1+p)X₄ is obtained. Specifically, if q mol among (1+p)mol of MeX₂ is substituted by MeX₃ in the above-mentioned method forsynthesizing Li_(2−2p)Me_(1+p)X₄, Li_(2−2p−q)Me_(1+p)X₄ is obtained.Also, if Me in Li_(2−2p−q)Me_(1+p)X₄ is chemically oxidized, alithium-containing halide represented by Li_(2−2p−q)Me_(1+p)X₄ isobtained.

Any of the above-mentioned lithium-containing halide acts as an activematerial for a lithium secondary battery.

Therefore, the lithium-containing halide of the present invention can berepresented by the formula: Li_(2−2p−q)Me_(1+p)X₄, wherein Me is atleast one transitionmetal element selected from the group consisting ofTi, V, Cr, Mn, Fe, Co, Ni and Cu, X is at least one halogen elementselected from the group consisting of F, Cl, Br and I, and 0≦p≦0.5 and0≦q≦2−2p.

As the Me, any one element among Ti, V, Cr, Mn, Fe, Co, Ni and Cu may beused alone, or two or more of them may be used in any combination. Inthe above-mentioned elements, Fe, Co or Mn is particularly preferable.

As the halogen in the lithium-containing halide of the presentinvention, fluorine, chlorine, bromine and iodine can be selected. Whenthe halogen is bromine or iodine, electrostatic interaction betweenthese halogen ions and a lithium ion becomes small since polarization ofthe halogen ion is large. That is, a lithium ion can move relativelyeasily in these halides, and the moving rate of a lithium ion in thesehalides becomes large. Consequently, a lithium secondary battery havingexcellent output current property can be obtained. Whereas, when thehalogen is chlorine or fluorine, the ion radius of the halogen ionbecomes small and a lithium secondary battery having high capacity canbe manufactured.

As the lithium ion conductive inorganic solid electrolyte in the presentinvention, those having high ion conductivity are preferably used forproviding high output of a battery. Among them, a sulfide-based lithiumion conductive inorganic solid electrolyte may advantageously be used.Sulfide-based amorphous (glassy) lithium ion conductive inorganic solidelectrolytes such as Li₂S—SiS₂, Li₂S—B₂S₃ and Li₂S—P₂S₅ are suitablesince they have high ion conductivity of not less than 10⁻⁴ S/cm.

Further, these sulfide-based lithium ion conductive inorganic solidelectrolytes are stable against a lithium-containing halide. Forexample, when a lithium halide and a sulfide-based lithium ionconductive inorganic solid electrolyte are heated at a high temperature,the lithium halide is incorporated into the structure of the solidelectrolyte to form a micro region composed of the lithium halide.

There is no change occurring in the crystal structure of the solidelectrolyte or in the micro structure itself of the lithium halide.Therefore, also when these lithium-containing halide and sulfide-basedsolid electrolyte are mixed, they dot not mutually react to deterioratetheir properties.

These sulfide-based lithium ion conductive inorganic solid electrolytesare generally synthesized by melting a mixture of starting materials ata high temperature and quenching the mixture. Among them, Li₂S—SiS₂manifests smaller transpiration of starting materials in synthesizing anelectrolyte since the vapor pressure of SiS₂ is higher as compared withB₂S₃ and P₂S₅. Accordingly, Li₂S—SiS₂ is most suitable for industrialmass synthesis.

In Li₂S—SiS₂-based solid electrolytes, lithium ion conductivity ismanifested when SiS₂ form an amorphous matrix and to this is added Li₂Sas a sulfide for modifying the matrix. Therefore, for manifesting higherion conductivity, a composition having a higher Li₂S content ispreferable. However, when the concentration of Li₂S is too high, thestability of the glass structure (matrix) lowers to the contrary andcrystallization occurs, leading to a decrease in ion conductivity.Therefore, it is preferable to mix starting materials at a mixing ratioindicated in the following exemplified method.

One example of the method for synthesizing a sulfide-based lithium ionconductive inorganic solid electrolyte will be illustrated in concrete.

As raw materials, 0.25 to 1 mol, preferably 0.4 to 0.7 mol of SiS₂,B₂S₃, P₂S₅ or the like is mixed with 1 mol of Li₂S. This mixture isheated at 700 to 1300° C., preferably 900 to 1100° C. for 1 to 12 hours,preferably 2 to 6 hours to obtain a melt (molten article). Finally, themelt is quenched. As the quenching method, there are listed a method inwhich the melt is applied to twin rollers to be drawn, and is solidifiedin a thinly drawn state, and the like. When the heating temperature ishigher than 1300° C., Li₂S and SiS₂ are thermally decomposed to changetheir compositions, and when lower than 700° C., the mixture is notmelted. Further, when the heating time is more than 12 hours, thecomposition changes significantly, and when less than 1 hour, thecomposition is not sufficiently melted and mixed.

Sulfurs in a Li₂S—SiS₂-based solid electrolyte are classified into across linking sulfur represented by the structure ≡Si—S—Si≡, and anon-cross linking sulfur represented by the structure ≡Si—S⁻. . . Li⁺.When a part of sulfurs in a Li₂S—SiS₂-based solid electrolyte issubstituted with oxygen, oxygen selectively substitutes for the crosslinking sulfur, to give a structure in which silicon atoms are connectedto cross linking oxygen (≡Si—O—Si≡).

Herein, each of “—” and “≡” represents a bond having a strong covalentbonding property, and “. . .” represents a bond having a strong ionbonding property. When silicon atoms are bonded via a cross linkingoxygen, the bond between silicon atoms becomes stronger and thestability of the glass structure becomes higher, as compared with thecase where silicon atoms are bonded via a cross linking sulfur.Consequently, it becomes possible to obtain an amorphous solidelectrolyte even if the content of the sulfide modifying the matrix israised. Then, a sulfide-based lithium ion conductive inorganic solidelectrolyte showing high ion conductivity can be obtained.

On the other hand, when the non-cross linking sulfur which contributesto bonding with a lithium ion is substituted by oxygen, theelectrostatic attractive force with a lithium ion becomes stronger.Namely, it becomes difficult for a lithium ion to move, and ionconductivity decreases. Therefore, it is preferable that the non-crosslinking site is occupied by sulfur.

From the above-described matters, as the sulfide-based inorganic solidelectrolyte having lithium ion conductivity, those having a crosslinking oxygen bonded to silicon atoms as described above areparticularly preferable. Such a sulfide-based lithium ion conductiveinorganic solid electrolyte having cross linking oxygen and siliconatoms bonded to the cross linking oxygen is obtained by producing amixture of lithium sulfide, silicon sulfide and for a oxygen source alithium oxide such as Li₂O or a lithium oxysalt such as Li₃PO₄ orLi₄SiO₄, as starting materials, and by melting and quenching themixture.

For example, in the above-mentioned one example of the method forsynthesizing a sulfide-based lithium ion conductive inorganic solidelectrolyte, a mixture prepared by mixing Li₂S with SiS₂, B₂S₃, P₂S₅ orthe like is allowed to further contain a lithium oxide such as Li₂O or asalt of lithium and an oxoacid such as Li₃PO₄ or Li₄SiO₄. By this, asulfide-based lithium ion conductive inorganic solid electrolyte havinga cross linking oxygen and silicon atoms bonded to the cross linkingoxygen is obtained. Herein, it is preferable that a salt of lithium andan oxoacid is contained in an amount of 0.005 to 0.1 mol, furtherpreferably 0.008 to 0.05 mol per 1 mol of Li₂S—M_(x),S_(y), wherein Mrepresents Si, B, P or the like.

Further, by controlling the content ratio of oxygen to silicone in theabove-mentioned sulfide-based lithium ion conductive inorganic solidelectrolyte to 40 to 60 mol per 100 mol of silicon, the amount of thecross linking oxygen bonded to silicon atoms can be made optimum.

The present invention will be further illustrated below using examples.

EXAMPLE 1

In this example, Li₂FeCl₄, which is a lithium-containing halide having aspinel structure was used as a positive electrode active material.Further, an amorphous inorganic solid electrolyte represented by 0.01Li₃PO₄-0.63 Li₂S-0.36 SiS₂ (indicating use of a mixture of 1 mol % ofLi₃PO₄, 63 mol % of Li₂S and 36 mol % of SiS₂, as a starting material)was used as a lithium ion conductive inorganic solid electrolyte, andmetallic lithium was used as a negative electrode active material. Alithium secondary battery was constituted as described below, and theproperties thereof were evaluated.

Li₂FeCl₄ was synthesized by the following method.

As the starting materials, lithium chloride (LiCl) and iron chloride(FeCl₂) were used. These were mixed at a molar ratio of 2:1, then, themixture was molded by press into a pellet, and sealed in a glass tubeunder a reduced pressure. This glass tube sealing the mixture ofstarting materials was heated at 500° C. for 3 days. The mixture wasthen ground in mortar to obtain Li₂FeCl₄.

Next, a sulfide-based lithium ion conductive inorganic solid electrolytewas synthesized by the following method.

Lithium phosphate (Li₃PO₄), lithium sulfide (Li₂S) and silicon sulfide(SiS₂) as starting materials were mixed at a molar ratio of 1:63:36, andthis mixture of starting materials was put in a crucible made of glassycarbon. This crucible was put into a vertical type furnace, and heatedat 950° C. in an argon flow, to make the mixture into the molten state.After heating for 2 hours, the melt was drawn by twin rollers andquenched to obtain a lithium ion conductive inorganic solid electrolyterepresented by 0.01 Li₃PO₄-0.63 Li₂S-0.36 SiS₂.

Thus obtained Li₂FeCl₄, a solid electrolyte, and fibrous graphite as anelectric conducting agent were mixed at a weight ratio of 50:48:2 toobtain appositive electrode material.

As the negative electrode, one obtained by punching a metallic lithiumfoil (thickness: 0.1 mm) into a size of 9.4 mm Φ was used.

The sectional view of a lithium secondary battery A in this example isshown in FIG. 1. In FIG. 1, numeral 1 represents a positive electrode,and the above-obtained positive electrode material weighed so that theweight of active materials was 50 mg was used. Numeral 2 represents theprepared lithium ion conductive inorganic solid electrolyte, and theelectrolyte 2 was molded by press integrally with the positive electrode1, followed by pressure-welding a negative electrode which is a metalliclithium foil 3 thereto. This integrally molded pellet was placed into astainless battery case 4, which was then sealed tightly with a stainlesslid 6 via an insulating gasket 5.

The characteristics of thus manufactured lithium secondary battery weremeasured by a charging and discharging test at a constant current of 50μA and voltages within a range from 1.5 to 4.5 V. The obtained chargingand discharging curve is shown in FIG. 2. As apparent from this figure,it is known that the lithium secondary battery manufactured inaccordance with the present invention manifests a battery voltage ofabout 3.5 V and is capable of charging and discharging.

EXAMPLE 2

In this example, a lithium-containing halide Li_(1.6)Fe_(1.2)Cl₄ havinga spinel analogous structure was used as a positive electrode activematerial instead of Li₂FeCl₄ obtained in Example 1. Further, anamorphous inorganic solid electrolyte represented by 0.01 Li₃PO₄-0.63Li₂S-0.36 SiS₂ was used as a lithium ion conductive inorganic solidelectrolyte, and metallic lithium was used as a negative electrodeactive material, to constitute a lithium secondary battery, and theproperties thereof were measured.

Li_(1.6)Fe_(1.2)Cl₄ was synthesized by the following method.

As the starting materials LiCl and FeCl₂ were used like in Example 1.These were mixed at a molar ratio of 1.6:1.2, then, the mixture wasmolded by press into a pellet, and sealed in a glass tube under areduced pressure. This glass tube sealing the mixture of startingmaterials was heated at 500° C. for 3 days. The mixture was then groundin mortar to obtain Li_(1.6)Fe_(1.2)Cl₄.

A lithium secondary batterys manufactured in the same manner as inExample 1 except that thus obtained Li_(1.6)Fe_(1.2)Cl₄ was used as apositive electrode active material, and the charging and dischargingproperties thereof were measured. As a result, the charging anddischarging capacity was lower as compared with that obtained in Example1, however the battery voltage was about 3.5 V, and it was confirmed tobe a chargeable and dischargeable battery.

Li_(1.6)Fe_(1.2)Cl₄ used in this example has a lower content of alithium ion as compared with Li₂FeCl₄ used in Example 1. The chargingreaction of this lithium secondary battery is a releasing reaction of alithium ion from Li_(1.6)Fe_(1.2)Cl₄. Therefore, the capacity appears tobe lowered since Li_(1.6)Fe_(1.2)Cl₄ used in this example contains asmaller amount of lithium ions.

EXAMPLE 3

In this example, a lithium-containing halide Li_(1.6)FeCl₄ having aspinel analogous structure was used as a positive electrode activematerial instead of Li₂FeCl₄ obtained in Example 1. Further, anamorphous inorganic solid electrolyte represented by 0.01 Li₃PO₄-0.63Li₂S-0.36 SiS₂ was used as a lithium ion conductive inorganic solidelectrolyte, and metallic lithium was used as a negative electrodeactive material, to constitute a lithium secondary battery, and thecharacteristics thereof were measured.

Li_(1.6)FeCl₄ was synthesized by the following method.

As the starting materials, LiCl, FeCl₂ and FeCl₃ were used. These weremixed at a molar ratio of 1.6:0.6:0.4, then, the mixture was molded bypress into a pellet, and sealed in a glass tube under a reducedpressure. This glass tube sealing the mixture of starting materials washeated at 500° C. for 3 days. The mixture was then ground in mortar toobtain Li_(1.6)FeCl₄.

A lithium secondary battery was manufactured in the same manner as inExample 1 except; that thus obtained L_(1.6)FeCl₄ was used as a positiveelectrode active material, and the charging and discharging propertiesthereof were measured. As a result, the charging and dischargingcapacity was lower as compared with that obtained in Example 1, however,the battery voltage was about 3.5 V, and it was confirmed to be achargeable and dischargeable battery.

Li_(1.6)FeCl₄ used in this example is alternatively represented byLi_(1.6)Fe²⁺ _(0.6)Fe³⁺ _(0.4)Cl₄ using the valency of Fe, and has alower content of Fe²⁺ as compared with Li₂FeCl₄ used in Example 1. Thecharging reaction of this lithium secondary battery is a reaction ofFe²⁺→Fe³⁺. Therefore, the capacity appears to be lowered sinceLi_(1.6)FeCl₄ used in this example contains a smaller amount of Fe²⁺.

EXAMPLE 4

In this example, a lithium-containing halide Li_(1.8)FeCl₄ having aspinel analogous structure was used as a positive electrode activematerial instead of Li₂FeCl₄ obtained in Example 1. Further, anamorphous inorganic solid electrolyte represented by 0.01 Li₃PO₄-0.63Li₂S-0.36 SiS₂ was used as a lithium ion conductive inorganic solidelectrolyte, and metallic lithium was used as a negative electrodeactive material, to constitute a lithium secondary battery, and thecharacteristics thereof were measured.

Li_(1.8)FeCl₄ was synthesized by the following method.

As the starting materials, LiCl, FeCl₂ and FeCl₃ were used like inExample 3. These were mixed at a molar ratio of 1.8:0.8:0.2, then, themixture was molded by press into apellet, and sealed in a glass tubeunder a reduced pressure. This glass tube sealing the mixture ofstarting materials was heated at 500° C. for 3 days. The mixture wasthen ground in mortar to obtain Li_(1.8)FeCl₄.

A lithium secondary battery was manufactured in the same manner as inExample 1 except that thus obtained Li_(1.8)FeCl₄ was used as a positiveelectrode active material, and the charging and discharging propertiesthereof were measured. As a result, the charging and dischargingcapacity was lower as compared with that obtained in Example 1, however,the battery voltage was about 3.5 V, and it was. confirmed to be achargeable and dischargeable battery.

Li_(1.8)FeCl₄ used in this example is alternatively represented byLi_(1.8)Fe²⁺ _(0.8)Fe³⁺ _(0.2)Cl₄ using the valency of Fe, and has alower content of Fe²⁺ as compared with Li₂FeCl₄ used in Example 1. Thecharging reaction of this lithium secondary battery is a reaction ofFe²⁺→Fe³⁺. Therefore, the capacity appears to be lowered sinceLi_(1.8)FeCl₄ used in this example contains a smaller amount of Fe²⁺.

EXAMPLE 5

In this example, a lithium-containing halide Li₂MnCl₄ having a spinelstructure was used as a positive electrode active material instead ofLi₂FeCl₄ obtained in Example 1. A lithium secondary battery wasconstituted in the same manner as in Example 1 except that anindium-lithium alloy was used as a negative electrode active material,and the characteristics thereof were measured.

Li₂MnCl₄ was synthesized by the following method.

As the starting materials, LiCl and MnCl₂ were used. These were mixed ata molar ratio of 2:1, then, the mixture was molded by press into apellet, and sealed in a glass tube under a reduced pressure. This glasstube sealing the mixture of starting materials was heated at 500° C. for3 days. The mixture was then ground in mortar to obtain Li₂MnCl₄.

A lithium secondary battery was manufactured in the same manner as inExample 1 except that thus obtained Li₂MnCl₄ was used as a positiveelectrode active material and a metallic indium foil (thickness: 100 μm)was used for a negative electrode, and a charging and discharging testwas conducted at voltages within a range from 2.0 to 4.0 V. As a result,this lithium secondary battery in accordance with the present inventionwas capable of charging and discharging between 2.0 and 4.0 V.

EXAMPLE 6

In this example, a lithium secondary battery was constituted in the samemanner as in Example 5 except that a lithium-containing halide Li₂CoCl₄having a spinel structure was used as a positive electrode activematerial, and the characteristics thereof were measured.

Li₂CoCl₄ was synthesized in the same manner as in Example 5 except thatLiCl and CoCl₂ were used as starting materials.

A lithium secondary battery was manufactured in the same manner as inExample 5 using thus obtained Li₂CoCl₄ as a positive electrode activematerial, and a charging and discharging test was conducted at voltageswithin a range from 2.0 to 4.0 V. As a result, this lithium secondarybattery in accordance with the present invention was capable of chargingand discharging between 2.0 and 4.0 V as in example 5.

EXAMPLE 7

In this example, a lithium secondary battery was constituted in the samemanner as in Example 5 except that a lithium-containing halide Li₂MnBr₄having a spinel structure was used as a positive electrode activematerial, and the characteristics thereof were measured.

Li₂MnBr₄ was synthesized in the same manner as in Example 5 except thatLiBr and MnBr₂ were used as starting materials.

A lithium secondary battery was manufactured in the same manner as inExample 5 using thus obtained Li₂MnBr₄ as a positive electrode activematerial, and a charging and discharging test was conducted. As aresult, this lithium secondary battery in accordance with the presentinvention was capable of charging and discharging.

EXAMPLE 8

In this example, a lithium secondary battery was constituted in the samemanner as in Example 1 except that a sulfide-based lithium ionconductive inorganic solid electrolyte represented by 0.05 Li₄SiO₄-0.60Li₂S-0.35 SiS₂ was used instead of the amorphous inorganic solidelectrolyte represented by 0.01 Li₃PO₄-0.63 Li₂S-0.36 SiS₂ used inExample 1 as an electrolyte, and graphite was used as a negativeelectrode active material, and the characteristics thereof weremeasured.

The sulfide-based lithium ion conductive inorganic solid electrolyterepresented by 0.05 Li₄SiO₄-0.60 Li₂S-0.35 SiS₂ was synthesized in thesame manner as in Example 1 except that a mixture obtained by mixinglithium orthosilicate (Li₄SiO₄), lithium sulfide and silicon sulfide ata molar ratio of 5:60:35 was used as the mixture of starting materials.

As the positive electrode material, a mixture obtained by mixing theabove-obtained solid electrolyte, Li₂FeCl₄ obtained in Example 1 and afibrous graphite material at a weight ratio of 48:50:2 was used. As thenegative electrode material, a mixture obtained by mixing theabove-obtained solid electrolyte and graphite at a weight ratio of 1:1was used.

100 Mg of the positive, electrode material and 50 mg of the negativeelectrode material were weighed, the solid electrolyte was placedbetween the positive electrode and negative electrode and they wereintegrally molded by press and then a lithium secondary battery wasmanufactured in the same manner as in Example 1, and the charging anddischarging properties thereof were measured. As a result, the workingvoltage of the lithium secondary battery of this example was about 3.4V, and it was confirmed to be a chargeable and dischargeable battery.

EXAMPLE 9

In this example, a lithium secondary battery was constituted in the samemanner as in Example 1 except that a sulfide-based lithium ionconductive inorganic solid electrolyte represented by 0.05 Li₂O-0.60Li₂S-0.35 SiS₂was used instead of the amorphous solid electrolyterepresented by 0.01 Li₃PO₄-0.63 Li₂S-0.36 SiS₂ used in Example 1 as anelectrolyte, and metallic indium was used as a negative electrode activematerial, and the characteristics thereof were measured.

The sulfide-based lithium ion conductive inorganic solid electrolyterepresented by 0.05 Li₂O-0.60 Li₂S-0.35 SiS₂ was synthesized in the samemanner as in Example 1 except that a mixture obtained by mixing Li₂O,lithium sulfide and silicon sulfide at a molar ratio of 5:60:35 was usedas the mixture of starting materials.

As the negative electrode, one that had been obtained by punching anindium foil having a thickness of 0.1 mm into a size of 9.4 mm Φ wasused, and an lithium secondary battery was manufactured in the samemanner as in Example 1.

In measuring the charging and discharging properties of this lithiumsecondary battery, the charging upper limit voltage was set to be 4.0 Vand the discharging lower limit voltage was set to be 1.0 V since thenegative electrode showed a potential of about 0.6 V based on a metalliclithium electrode. As a result, the working voltage of the lithiumsecondary battery of this example was about 3.0 V, and it was confirmedto be a chargeable and dischargeable battery.

EXAMPLE 10

In this example, a lithium secondary battery was manufactured in thesame manner as in Example 1 except that a sulfide-based lithium ionconductive inorganic solid electrolyte represented by 0.6 Li₂S-0.4 SiS₂was used instead of the amorphous solid electrolyte represented by 0.01Li₃PO₄-0.63 Li₂S-0.36 SiS₂ used in Example 1 as an electrolyte, and thecharging and discharging properties thereof were measured. As a result,the lithium secondary battery of this example also showed approximatelythe same properties as those in Example 1.

EXAMPLE 11

In this example, a lithium secondary battery was manufactured in thesame manner as in Example 1 except that a lithium ion conductiveamorphous solid electrolyte represented by 0.6 Li₂S-0.4 P₂S₅ was usedinstead of the amorphous solid electrolyte represented by 0.01Li₃PO₄-0.63 Li₂S-0.36 SiS₂ used in Example 1 as an electrolyte, and thecharging and discharging properties thereof were measured. As a result,the lithium secondary battery of this example also showed approximatelythe same properties as those in Example 1.

EXAMPLE 12

In this example, a lithium secondary battery was manufactured in thesame manner as in Example 1 except that a lithium ion conductiveamorphous solid electrolyte represented by 0.5 Li₂S-0.5 B₂S₃ was usedinstead of the amorphous solid electrolyte represented by 0.01Li₃PO₄-0.63 Li₂S-0.36 SiS₂ used in Example 1 as an electrolyte, and thecharging and discharging properties thereof were measured. As a result,the lithium secondary battery of this example also showed approximatelythe same properties as those in Example 1.

EXAMPLE 13

In this example, a lithium secondary battery was manufactured in thesame manner as in Example 1 except that a sulfide-based lithium ionconductive inorganic solid electrolyte represented by 0.30 LiI-0.35Li₂S-0.35 SiS₂ was used instead of the amorphous solid electrolyterepresented by 0.01 Li₃PO₄-0.63 Li₂S-0.36 SiS₂ used in Example 1 as anelectrolyte, and the charging and discharging properties thereof weremeasured. As a result, the lithium secondary battery of this examplealso showed approximately the same properties as those in Example 1.

EXAMPLE 14

In this example, a lithium secondary battery was constituted using thelithium-containing chloride having a spinel structure represented byLi₂FeCl₄ obtained in Example 1 as a negative electrode active material,an amorphous solid electrolyte represented by 0.01 Li₃PO₄-0.63 Li₂S-0.36SiS₂ as a lithium ion conductive inorganic solid electrolyte and LiCoO₂as a positive electrode active material, and the properties thereof weremeasured.

The positive electrode active material LiCoO₂ was synthesized by thefollowing method.

As the starting materials, cobalt oxide (Co₃O₄) and lithium carbonate(Li₂CO₃) were used. These were weighed and mixed at a molar ratio ofCo/Li=1, then, the mixture was calcined at 900° C. in atmosphere, tosynthesize LiCoO₂.

Thus synthesized LiCoO₂ was mixed with the solid electrolyte powder at aweight ratio of 6:4, to give a positive electrode material.

As the negative electrode material, the mixture of Li₂FeCl₄ obtained inExample 1, the solid electrolyte and fibrous graphite was used.

150 Mg of positive electrode material and 50 mg of negative electrodematerial thus obtained were weighed, the solid electrolyte was placedbetween the positive electrode and negative electrode and they wereintegrally molded by press and a lithium secondary battery wasmanufactured in the same manner as in Example 1, and the charging anddischarging properties thereof were measured. Charging and dischargingat voltages within a range from 0 to 3.5 V indicated that this lithiumsecondary battery was a chargeable and dischargeable battery.

Comparative Example 1

For comparison, a lithium secondary battery was manufactured using aliquid electrolyte which is a non-aqueous solvent electrolyte as anelectrolyte, Li₂FeCl₄ obtained in Example 1 as a positive electrodeactive material, and metallic lithium as a negative electrode activematerial.

As the non-aqueous solvent electrolyte, two kinds of electrolytes wereused: one obtained by dissolving lithium hexafluorophosphate (LiPF₆) ata concentration of 1 M (mol/liter) into a solvent prepared by mixingethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a voluminalratio of 1:1, and one obtained by dissolving LiPF₆ at a concentration of1 M into propylene carbonate (PC).

To Li₂FeCl₄ obtained in Example 1 was added 5% by weight of fiberousgraphite as a conducting agent and further 5% by weight of afluorocarbon resin as a binder, and mixed. This mixture was weighed sothat the weight of Li₂FeCl₄ in the mixture was 50 mg, and filled in amesh of 9.4 mm Φ made of high chromium stainless steel, to give apositive electrode pellet.

This positive electrode pellet, a metallic lithium foil having athickness of 0.34 mm as a negative electrode, and a polypropylene porousfilm having a thickness of 50 μm as a separator were used to constitutea lithium battery having a cross-section as shown in FIG. 3, using theabove-described non-aqueous solvent electrolyte. In FIG. 3, numeral 7represents the positive electrode pellet, numeral 8 represents theseparator, numeral 9 represents the negative electrode, and numeral 10represents a battery case of stainless steel, and the non-aqueoussolvent electrolyte 11 was poured into the case, then, the case wassealed with a cover 13 via a gasket 12.

The charging and discharging properties of thus manufactured lithiumsecondary battery were evaluated in the same manner as in Example 1. Asa result, a charging and discharging efficiency of as low as not morethan 70% was shown, and reduction in capacity concurrent with chargingand discharging cycle was remarkable. The battery was decomposed toinvestigate this reason, and deposition of metallic iron was observed onthe metallic lithium of the negative electrode, and dissolution ofLi₂FeCl₄ used as a positive electrode active material into thenon-aqueous electrolyte was supposed to be the reason.

In the examples of the present invention, explanations were made only onthe cases in which Li₂FeCl₄, Li₂MnCl₄ and the like are used as thelithium-containing halide having a spinel structure or a spinelanalogous structure. However, also when other lithium-containing halideshaving a spinel structure or a spinel analogous structure are used,excellent lithium secondary batteries can be constituted likewise.

Further, in the examples of the present invention, explanations weremade only on the cases in which the lithium-containing halide having aspinel structure or a spinel analogous structure was used for either apositive electrode active material or negative electrode activematerial. However, since these halides show high reversibility in alithium secondary battery using a lithium ion conductive inorganic solidelectrolyte as an electrolyte, a lithium secondary battery can also bemanufactured using them in both of the positive electrode and negativeelectrode.

Furthermore, in the examples of the present invention, explanations weremade only on the cases in which Li₂S—SiS₂ system and the like were usedas the lithium ion conductive inorganic solid electrolyte. However, thesame effect was obtained when other solid electrolytes such asLi₂S—Al₂S₃and the like were used, when other sulfide-based lithium ionconductive inorganic solid electrolytes such as LiBO₂—Li₂S—SiS₂ and thelike were used as the sulfide-based lithium ion conductive inorganicsolid electrolyte having a cross-linking oxygen and silicon ions bondedto the cross-linking oxygen ion, further, when oxide-based lithium ionconductive inorganic solid electrolytes were used.

INDUSTRIAL APPLICABILITY

In accordance with the present invention, by use of a lithium ionconductive inorganic solid electrolyte as an electrolyte, a chargeableand dischargeable lithium secondary battery in which an electrode activematerial is a lithium-containing halide having a spinel structure or aspinel analogous structure can be obtained.

What is claimed is:
 1. A lithium secondary battery comprising achargeable and dischargeable positive electrode, a chargeable anddischargeable negative electrode and a lithium ion conductive inorganicsolid electrolyte, wherein at least one of said positive electrode andsaid negative electrode comprises a lithium-containing halide having aspinel structure or a spinel analogous structure.
 2. The lithiumsecondary battery in accordance with claim 1, wherein saidlithium-containing halide is represented by the formula:Li_(2−2p−q)Me_(1+p)X₄ where Me is at least one transition metal elementselected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Cu,X is at least one halogen element selected from the group consisting ofF, Cl, Br and I, and 0≦p≦0.5 and 0≦p≦2−2p.
 3. The lithium secondarybattery in accordance with claim 2, wherein said lithium-containinghalide is represented by Li₂MeX₄ where Me is Fe, Mn or Co.
 4. Thelithium secondary battery in accordance with claim 1, wherein saidlithium ion conductive inorganic solid electrolyte is a sulfide-basedlithium ion conductive inorganic solid electrolyte.
 5. The lithiumsecondary battery in accordance with claim 1, wherein said lithium ionconductive inorganic solid electrolyte is a sulfide-based lithium ionconductive inorganic solid electrolyte containing silicone and oxygen,an amount of said oxygen being 5 to 70 mol per 100 mol of silicon.